Vacuum systems that can generate high vacuums are used for many purposes such as semiconductor manufacture, research experimentation in companies and universities, etc. Such vacuum systems often have expensive turbo pumps, ionization gauges, getter pumps, diffusion pumps, substrates and other materials which are susceptible to damage from any component of air such as oxygen or water vapor. For example, oxygen in inrushing air and reacts with heated surfaces or other heated materials and oxidizes them. To repair or replace such pumps and other equipment can cost thousands to millions of dollars depending upon how complicated the system is.
Many such vacuum systems have viewing ports through which the experimenter or engineer interested in the process going on inside the vacuum system can look. Typically, these viewing ports are round metal structures surrounding a transparent pane of glass or other electromagnetically transparent material. The pane of glass or other material must has sufficient strength to withstand the pressure on the pane from the atmosphere based upon the pressure differential between the atmospheric pressure and the much lower pressure in the vacuum chamber. If one could create a perfect vacuum, the atmospheric pressure acting on the pane would be 14.6 psi. The transparent pane of the viewing port is mounted within a metal structure and hermetically sealed so as to seal the vacuum in. Typically these metal structures are round ports, and only one pane of glass or other material is used with the diameter of the port being from 2.75 inches to 4 inches in diameter. Some ports can be as large as 10 inches in diameter. Thicknesses of the pane vary depending on the diameter of the port and are between 1 millimeter and 6.5 millimeters thick with 4 mm being typical.
Some applications for vacuum systems require a laser beam to be shined through the viewing port. Such applications include at least the following.                1) Pulse laser deposition. This is a thin film deposition technique where a high power laser beam is focused inside a vacuum chamber to strike a target of the desired composition. Material vaporized from the target is deposited as a thin film on a substrate.        2) Laser driven particle acceleration: One technique for particle acceleration is crossing laser beams in a vacuum. This causes longitudinal electric fields in the laser pattern to accelerate electrons in an electron beam directed into the intersection of the laser beams.        3) Laser induced breakdown emission spectroscopy: This is a technique used to determine the elements in solids, liquids and aerosols. The method requires little or no sample preparation time and involves focusing a laser beam on a sample and collecting the light emitted from the sample as a result of the laser excitation. This is sometimes done in a vacuum to remove background signals caused by light emitted from excited substances other than the sample being analyzed.        4) Matrix assisted laser desorption mass spectrometry: This is a soft ionization technique used in mass spectrometry allowing the analysis of biomolecules such as proteins and peptides and sugars as well as large organic molecules such as polymers, etc. which tend to be fragile and to fragment when ionized by more conventional ionization methods. The ionization I triggered by a laser beam which is focused on a matrix which protects the biomolecules from being destroyed by direct contact with the laser beam. The matrix facilitates the vaporization and ionization. Typically the matrix is sinipinic acid.        
The problem with directed high energy laser beams through a pane of transparent material is that such an act can cause destruction of the pane and breach of the vacuum integrity of the viewing port. When the pane is destroyed, fragments of the material can enter the vacuum chamber and be swept into the intake of the turbo vacuum pump(s). Debris ingestion can destroy or damage such pumps and engender high repair costs. The failure mechanisms are varied, but can involve skin oil smudges on the transparent pane or dirt or dust which can cause uneven localized heating to very high temperatures in some spots compared to cooler regions of the pane adjacent to the spots which have been unevenly heated. This can rupture the glass or cause a crack to start caused by rapid thermal expansion of one section of the transparent material relative to a cooler region adjacent the hot region. A crack can eventually or immediately cause the pane to fail because of the forces acting on the pane because of the pressure difference between the vacuum in the chamber and the atmospheric pressure pushing on the pane from the outside.
In addition, high pressure systems need viewing ports and they need to be fracture safe to keep particles from being blown into the eye of an observer. Fractures can start either from a mechanical blow or unequal heating caused by passing of a laser or other electromagnetic beam through the pane of a viewport. Fractures can be dangerous and expensive and are to be avoided.
A fracture-safe viewing port through which a laser beam or other electromagnetic beam of high energy can be directed into a pressure system chamber without causing breach of the vacuum or pressure integrity of the chamber is therefore needed.